APL keyboards
APL is a programming language which is mostly typed with special symbols rather than words. In its early life it was written on printing terminals fitted with an APL typeball and the symbols printed on the keyboard:
IBM 2741 Communications Terminal. Source: Wikipedia
APL keyboard layout. Source: Wikipedia
Today you don't need a special keyboard to program in APL but should you want one there are two off-the-shelf ones available that I know of. One from Dyalog and Cherry that uses MX Black switches and has US, UK and DE layouts available:
Cherry G80-3000LPCXY-12 with APL glyphs. Source: Dyalog
And one from Unicomp with buckling springs but only in US layout. APL was primarily developed for IBM systems and they helped bring it to the market, and so in the 80s APL versions of their Model M keyboard were produced. When Model M production was sold to Lexmark and later Unicomp, they kept making these:
Unicomp Ultra Classic US APL Black Buckling Spring 104 Key USB Keyboard. Source: Unicomp
NovelKeys and CannonKeys both make APL Cherry keycap sets with ANSI and ISO keys included:
Cherry Olivia PBT Dyesub Keycaps. Source: NovelKeys
CannonCaps Big Blue. Source: CannonKeys
Of course many APL enthusiasts have built their own custom keyboards:
Source: xera-vv on Reddit
Source: JoeLeeYN on Reddit
Source: EST4R on Reddit